1. Technical Field
The invention relates to a plant for storing energy by means of compressed air.
2. Prior Art
Initial State:
Fossil energy is to be replaced (owing to CO2) with renewable energy. In the field of electricity supply, difficulties are posed by the fact that the availability of renewable energy, such as wind—utilized by means of wind turbines—and sun—utilized by means of photovoltaics—fluctuates greatly and the demand for electrical energy can be substantially covered by renewable energy only if the electricity generated thereby can be stored. Known methods for this are storage batteries—presently too expensive—pumped-storage power plants—these require relatively large height differences and do not exist for example in the North German lowlands, and furthermore often elicit public protests owing to “landscape degradation”—or compressed-air storage plans (often referred to in the literature as CAES—“Compressed Air Energy Storage”).
These can store electrical energy on a large scale and, for this purpose, use underground cavities, large turbocompressors and turbines. Existing plants operate with the compressed air being heated, by means of natural gas combustion, before expansion in order to prevent the formation of ice in the turbine, and a further advanced approach (normally referred to as adiabatic compressed-air storage power plant or “adiabatic CAES”) has the aim of storing the heat of compression released during the compression and releasing said heat again into the compressed air before the latter is expanded in the turbine. In this way, it is possible even in flat areas to store a very large amount of energy for electricity supply, without the landscape being changed, without the use of additional fossil energy, with good storage efficiency of approximately 70%, and at very low cost. The costs for the operation of such a plant are almost exclusively capital costs. And with greater use of renewable energies, these will only fall further.
This method is described in the documentation of the “ADELE” project, but something similar was planned as early as in the 1980s (→CAES Studies Pacific Northwest Labs 1981.pdf, Conceptual Design and Engineering Studies of Adiabatic Compressed Air Energy Storage (CAES) with Thermal Energy Storage, M. J. Hobson et. al., November 1981, Pacific Northwest Laboratory—Batelle, PNL-4115). In the ADELE project, technically high targets were set, with pressures of up to 100 bar and temperatures up to 600° C. in the heat store.
FIG. 1 describes the schematic construction of a known adiabatic compressed-air storage power plant. A compressed-air store is provided in a volume which is normally underground. To fill said compressed-air store, ambient air is introduced into the volume DS from atmospheric pressure p-atm, and through a heat store WS in which the air releases its heat of compression to a storage material, by means of a compressor K. Only low flow pressure losses are generated in the heat exchanger WS. The direction of the air is indicated by the dashed arrows. Here, an electric machine M/G, which can operate as a motor and as a generator, drives the compressor K via a switching coupling SK1 which is closed for this purpose. If the stored energy is to be called upon, air flows out of the compressed-air store DS through the heat store WS, where the stored heat of compression is substantially transferred back again, into a turbine T, and finally into the atmosphere. This is indicated by the solid arrows. The turbine T is connected to the electric machine M/G via a second switching coupling SK2, which is closed for this purpose, said electric machine now operating as a generator. Valves V ensure the necessary opening and closing processes.
The Object:
In order that said method of compressed-air storage can solve, with even greater effectiveness, the problem of homogenizing the availability of renewable energy, it must become cheaper. If gaps in renewable energy availability lasting for relatively long periods of time, for example for several days, are to be compensated for, it must be possible to build up storage capacity which is utilized not daily but rather a few times in a year and which, in the process, is depreciated. This is possible through improved utilization of a storage volume (at the cost of X Euro per m3 geometric volume) with higher pressure and greater pressure fluctuations.
The same problem is encountered if it is sought to construct small stores in which the expensive initial investment of many millions of Euros for a bore into the ground is uneconomical, and which are thus dependent on small overground stores. Here, too, the capital costs of the plant per stored kWh of capacity are lower the higher the pressure and the higher the pressure fluctuations in the store are. Therefore, with regard to the existing projects, the following would still appear to have room for improvement:
If an underground storage volume is filled sometimes to a greater extent and sometimes to a lesser extent, then one encounters a fluctuating pressure therein (considerations have been made regarding pressure compensation using water, described for example in DE102007042837A1 and also in many earlier sources, but this was not realized owing to the difficulties associated with the water) and in the turbomachines that compress or expand compressed air, the flow conditions deviate from the design point, resulting in losses in efficiency. Therefore, the pressure fluctuations are limited to low values, the storage capacity of the volume is thus only partially utilized, more volume is thus required, and costs are higher.
In the cited DE102007042837A1, it is the intention for a high-pressure compressor designed specifically for fluctuating pressure to eliminate the problem using precisely such a turbine; another approach would be rotational speed regulation of the turbomachines in order, similarly to the approach with pump turbines of regulable rotational speed in pumped-storage power plants, to keep the velocity triangles approximately congruent and thus efficiency losses low—said approach then however has the disadvantage of the costs for cumbersome power electronics in the megawatt range and can only with difficulty provide the demand for so-called short-circuit current, as demanded in conventional electricity grids.
In a solution with only turbomachines, it is also difficult to realize high pressures for the good utilization of the storage volume, in particular in the case of less than 50 megawatt plant power.
The concept of the “pump turbine”, that is to say the utilization of the same radial turbomachine both for pumping and for turbine operation (as realized in relatively new pumped-storage power plants, for example Goldisthal), is also already applied to compressed-air stores in JP000004347335A. Thus, a second possibility for improvement of the hitherto planned concepts is mentioned: capital costs can be lowered if one utilizes the same turbomachine both for compression and expansion.
Room for improvement also exists with regard to the starting time of a conventional compressed-air storage power plant. Owing to the high temperature fluctuations, there is a relatively long time between standstill and running at full load.